This invention relates generally to a servo steering gear, and particularly for an alleviator controlled, constant pressure servo system for motor vehicles. More particularly, the invention comprises an input member operatively connected to a steering wheel and an output member operatively connected to the steered wheels, a servo motor operatively connected to the output member, control means for the pressure fluid and having connections for a pressure pipe to a pressure souce particularly in the form of a pump associated with an alleviator, for an exhaust pipe and for at least one working pipe leading to one working chamber of the servo motor, actuating means for the valve elements of the control means for controlling the servo motor, said actuating means responding to relative motion between input and output member not resulting in a direct transmission of force, and said valve elements being shiftable by the actuating means into a position in which a working chamber of the servo motor is connected to the pressure source or to exhaust to assist the motion of the output member initiated by the input member. The valve elements are so disposed that an automatic restoration of the steered wheels can take place and that pressure-loaded reaction faces provided on the valve elements generate a reaction which the vehicle driver can feel, and which at least within a defined range depends upon the reactive forces generated by the steered wheels.
Recently, there has been increasing support for the idea of replacing the hitherto conventional "open center" piston valves in the control means for controlling the servo motor by piston valves of the "closed center" type. The reason is that servo steering gear availing itself of "closed center" piston valves can be supplied by the general hydraulic system of the vehicle, more particularly, alleviators can be incorporated as hydraulic storage devices and safety reservoirs, and smaller capacity pumps can be used. Naturally, it has been the aim to retain the advantages of "open center" valves. The latter comprise automatic restoring action of the steered wheels into the neutral position, a predetermined initial pressure in the working chambers of the servo motor in neutral position, the possibility of providing direct mechanical steering within a limited range about the neutral position for the purpose of providing a response threshold, and finally, the possibility of limiting the steering effort to a maximum value.
Two alternatives in principle have been proposed for solving these problems. In the first alternative a spring-loaded control valve is associated with each working chamber of a servo motor. In the other alternative use is made of the so-called two-edge control system which avails itself of a servo motor having different hydraulic cross sections. The smaller cross section is permanently loaded by the pressure supplied by the source, whereas the pressure fluid supplied to or exhausted from the working chamber having the major hydraulic cross section is controlled by a piston valve having two control edges. Whereas forms of construction comprising two control pistons located in parallel or a single piston valve disposed parallel to a reaction piston have already been successful (German Patent Applications Nos. P 22 42 021.9, P 22 42 022.0), difficulties arise when in a two-edge control system two reaction faces are to be provided on one piston valve or when in a four-edge control system both piston valves are to be coaxially arranged. Since in such cases some degree of displaceability of the control piston or pistons, which is independent of the actuating member, must be provided in order to permit automatic restoration of the steered wheels to neutral position, the actuating member has been coupled to a supplementary piston which in turn acts on the control piston or pistons through interposed springs. Such arrangements are described for instance in copending USSN 391,271, filed Aug. 4, 1973 and USSN 471,699, filed May 20, 1974; and in U.S. Pat. No. 3,927,602. In order to avoid unacceptably great overall lengths of the steering gear it was necessary to make the supplementary piston relatively short. However, in the case of an eccentrically positioned actuating member, such as a steering finger coupled to a steering nut, this introduces the risk of jamming and excessive hystoresis effects. These arrangements were not therefore desirable when short overall length units were required.